Boom and feed line construction for multielement antenna



June 30, 1970 W.-F. SCHICK ETAL 3,518,590

BOOM AND FEED LINE CONSTRUCTION FOR MULTIELEMENT ANTENNA Filed Dec. 1, 1966 3 Shets -Sheet z.

INVENTORS I MARVIN L. WAHL y WILLIAM E SCHICK WILLIAM R. LA VALLEY CLAES 1'. ELFVING BY M ATTORNEY June 3o, .1970

Wu F scI-IIcK ETAL; 1 3,518,690

BOOM AND FEED LINE CONSTRUCTION FOR MULTIELEMENT ANTENNA I I I 0 i1 wk: v :.:I ink, I A --..,;I- y I ""imii'n wwlialai MARVIN I.. wAI-IL I WILLIAM F. SCHICK g o n h n O m WILLIAM R. LA VALLEY o n 5 T CLAES T. ELFVING BY M h I ATTQRNEY W. F- SCHICK ETAL Jun: 30, 1970 BOOM AND FEED LINE CON$TRUCTION FOR MULTIELE'MENT ANTENNA Filed Dec. 1, 1966 3 Sheets-Sheet 5 A r AEVIIIIIA: VII:

INVENTORS MARVIN L WAHL WILLIAM F. SCHICK WILLIAM R. LA VALLEY CLAES T. ELFVING ATTORNEY United States Patent US. Cl. 343-7925 11 Claims ABSTRACT OF THE DISCLOSURE A boom for supporting the radiator elements of a log periodic antenna is constructed as a unitary tubular structure comprising two or more flat or angled conductor strips electrically insulated from each other. The elements are directly mechanically and electrically connected to the flat surfaces of the strips which conduct electrical energy between the elements and external feed lines. The dimensions of strips may change uniformly over the length of the antenna to provide a tapered boom. The strips are mechanically interconnected by electric insulators and have a predetermined spacing between adjacent edges for optimum line impedance. Feed lines within the boom are connected to the plates so as to provide a balanced feed for the elements on opposite sides of the boom.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to log periodic antennas, and more particularly to the construction of the boom to which the antenna elements are connected.

Description of the prior art Log periodic antennas utilizing dipole arrays are generally physically elongated structures with the antenna elements projecting radially from and spaced along the longitudinal axis of the antenna. Typical log periodic antenna structures of this design are described in Pat. Nos. 3,113,- 316 and 3,123,827. The dipole elements must be rigidly supported along the axis of the antenna in order to occupy fixed spatial positions and also must be coupled to feed lines over the length of the antenna in such a manner that complementary halves of each dipole are fed 180 out-of-phase with each other and adjacent dipoles are also fed 180 out-of-phase with each other. In the past, accomplishment of these two functions, mechanical support and electrical feed, has generally resulted in relatively complicated unwieldy structures that are costly to build and difiicult to assemble.

Assembly of such antennas is further complicated by the need for efficient electrical coupling between the feed lines and the elements as well as rigid mechanical support of the elements. If the elements are capacitively coupled to the feed line, antenna assembly procedure is time consuming and difficult because of the criticality of spacing between each element and the line. If the elements are directly electrically connected to the feed lines by brazing, soldering, etc., the difiiculty of fabricating such arrays in the field renders impractical the use of a collapsible antenna design to facilitate assembly and disassembly of large antennas for convenience in transporting and handling them.

An additional factor in the construction of antennas of the type described above is the desirability of uniformly tapering the spacing between the element feed lines over the length of the antenna. Such tapering, which is illustrated in Pat. No. 3,123,827, improves broad- 3,518,690 Patented June 30, 1970 ice band performance of the antenna by insuring proper spacing of the center fed dipoles over the band. Tapering imposes an additional constructional constraint on the log periodic antenna which has complicated its construction and has added to its cost.

A general object of this invention is the provision of low-cost log periodic antenna that retains performance characteristics of higher cost designs.

Another object is the provision of a log periodic antenna with a unitary boom for mechanically supporting and electrically feeding the antenna elements.

A further object is the provision of an antenna of this type which may be readily prefabricated in separate easily transportable units for ready assembly in the field.

Still another object is the provision of a combination boom and feed line assembly made of relatively simple structural shapes that may be easily tapered to provide a tapered feed line for the antenna.

SUMMARY These objects are accomplished with a unitary boom and feed line assembly comprising an elongated rectangular tube having an axis coincident with the longitudinal axis of the antenna and made with flat-sided electrically conductive strips mechanically secured together but electrically insulated from each other. The fiat sides of the strips provide ample and conveniently shaped surfaces to which the dipole elements may be efficiently removably connected, both mechanically and electrically, with standard tools in the field. The electrically conductive boom strips are separate feed lines and as such are precisely spaced apart at their adjacent edges by insulators so that the characteristic impedance of these feed lines remains substantially constant over the entire length of the boom. The dipole elements are removably secured directly to the respective strips at predetermined axial positions in accordance with the logarithmically periodic relationship.

In one embodiment having vertically and horizontally polarized dipole sets, four plate-like strips are used as two pairs of balanced lines which pairs are energized by two coaxial lines extending longitudinally within the boom. In another embodiment having dipoles polarized on one plane, a pair of opposed angle-shaped strips are supported so as to define a rectangular tube and are fed by a single coaxial line disposed within and coextensive with a substantial part of the boom. In either embodiment, longitudinal tapering, i.e., uniform dimensional change of the boom from one end to the other, is readily and economically achieved because the plane or angled shapes of the strips permit conventional metal forming techniques to be employed for this purpose.

DESCRIPTION OF THE DRAWINGS These and other objects of the invention will become apparent from the following description of a preferred embodiment thereof, reference being had to the accompanying drawings in which:

FIG. 1 is a perspective view of a log periodic antenna with horizontally and vertically polarized dipole arrays and a boom constructed in accordance with this invention;

FIG. 2 is an enlarged longitudinal section of one end of the antenna of FIG. 1;

FIG. 3 is a transverse section taken on line 33 of FIG. 2; and

FIG. 4 is a section similar to FIG. 3 showing a modilied form of the invention in which the boom is made of angled strips.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, FIG. 1 shows a log periodic antenna 10 comprising an elongated longitudinal extending boom 12 supporting a vertical set 13 and a horizontal set 14 of dipole elements. As shown, boom 12 is mounted with its axis L extending horizontally and is supported at one end on a frame 15 through a dielectric bracket 16 and by a bipod 17 connected through a dielectric adapter 18 to the boom intermediate its ends. It. will be understood that the terms vertical and horizontal are descriptive of these parts of an antenna in but one of many operative positions, namely when the antenna is vertically and horizontally polarized as shown in the drawing. The terms vertical and horizontal, however, are intended merely to define relative positions of the two dipole sets and the boom for a typical antenna orientation and are not to be construed as limiting terms. The axis L of the boom, for example, might be oriented to extend vertically with both sets of dipoles lying in horizontal planes.

Both sets 13 and 14 of dipole elements project fro the boom in planes perpendicular to axis L with element lengths and interelement spacings increasing from a minimum at end A of the antenna to a maximum adjacent end B thereof in progressive increments at a predetermined ratio that defines the well-known characteristics of a logarithmically periodic antenna. In accordance with this invention, the boom itself comprises balanced feed lines for all of the dipole elements and since the spacing between the feed lines of each pair varies uniformly over the length of the antenna, the cross-sectional dimension of boom 12 changes uniformly, i.e., the cross-sectional dimension of the boom tapers from a minimum at end A to a maximum at end B. The vertical and horizontal dipoles which operate at the same wavelength are connected to the boom at the same axial location.

Boom 12 comprises four longitudinally extending plane electrically conductive strips or plates 20, 21, 22, and.23, see FIGS. 2 and 3, arranged symmetrically about axis.L in opposed pairs with one pair of strips 20, 22 lying in planes perpendicular to the planes of the other pair of strips 21, 23. These strips are mechanically connected along their adjacent longitudinal edges and are precisely spaced apart by a distance d by means of longitudinally extending insulator brackets 25 having a right-angled cross-section as shown. Brackets 25 may be longitudinally continuous or may comprise several longitudinally spaced parts. Strips 20-23 are mechanically secured to the insulator brackets by any suitable means such as screws, rivets or, as shown in FIG. 3, by cement to form a unitary rectangular or square tubular structure which is rigid and mechanically self-supporting.

The vertical set 13 of dipoles are connected to strips 21 and 23 while the horizontal set 14 of dipoles are connected to strips 20 and 22. More specifically, elements 13a and 13b of each vertically polarized dipole and elements 14a and 14b of each horizontally polarized dipole are mounted directly on the respective strips in such a manner that the elements may readily be collapsed or removed entirely from the boom. Thus, the elements may be secured to the strips by clamps 27 and screws 28 or may be pivotally mounted thereto for movement between collapsed and extended positions.

In 10g periodic antennas using dipole arrays, adjacent dipoles along the axis of the antenna typically are fed 180 out-of-phase with each other in order to propagate (or receive) electromagnetic waves unidirectionally, that is, along the axis of the antenna. In accordance with this invention, this phase reversal of adjacent dipole elements in each set is conveniently achieved by simply reversing the direction of projection of adjacent dipole elements connected to the same longitudinal strip of the boom. For example, element 13a shown in FIG. 3 extends upward from boom strip 21 and each longitudinally adjacent dipole element connected to strip 21 extends downwardly therefrom. Since opposed strips 21 and 23 as well as strips 20 and 22 are electrically conductive members and are connected to the dipole elements of each set, each pair of strips comprises a balanced feed line for one set of dipoles as well as principal structural members of the boom.

In order to electrically connect the antenna to external circuits, coaxial lines 30 and 31 are disposed within and extent longitudinally of the boom from end B to end A thereof. The outer conductor 30a of line 30 is electrically connected to the inside surface of conductive strip 21 by suitable means such as clamps 33 Which press the uninsulated outer conductor directly against the conductive strip. Similarly, the outer conductor 31a of coaxial line 31 is connected by clamp 34 directly against the inner surface of conductive strip 22 of the boom.

The inner conductor of each coaxial line is electrically connected to the conductive boom strip opposite from the strip to which the outer conductor of that line is connected. Thus, the inner conductor 30b of coaxial line 30 is connected to conductive strip 23 and inner conductor 31b of line 31 is connected to conductive strip 20. In order to maintain uniform line impedance at these connections and to provide a proper match between the boom and the coaxial lines, conductive strips 20, 21, 22, and 23 of the boom have inwardly projecting trapezoidally shaped conductive terminals 36, 37, 38, and 39, respectively, electrically connected thereto at the smaller end ened A of the boom. These terminals are configured and so positioned as to be equally spaced apart by a distance d and inner conductors 30b and 3112 project from the outer conductors 30a and 31a, respectively, of the respective coaxial lines at the longitudinal axis of the boom, see FIG. 3, for connection to the extension of the opposite conductive strip. A dielectric cap 40 over the end of the boom protects these connections from the elements.

In order that each dipole in the array be properly energized at the design frequency of the dipole, the spacing between the center feed points of the several dipoles is varied progressively over the length of the antenna. Therefore the spacing between opposed conductive strips of each pair changes uniformly from a minimum adjacent end A of the antenna to a maximum at the opposite end B. This change of dimension over the length of the antenna, called tapering, is achieved in the boom construct ed in accordance with this invention simply through a progressive change in the width of each conductive strip along the antenna axis. The rectangular or square cross-sectional configuration of the boom is retained and though this cross-sectional dimension changes uniformly over the length of the antenna, there is no sacrifice of strength or rigidity nor any undue complication in the supporting of the conductive strips relative to each other. If the operating frequency range of the antenna is sufficiently limited, however, say an octave or less, tapering of the boom is not required.

The characteristic impedance of each pair of strip transmission lines forming the boom is an important factor in coupling energy to the dipole elements. In order to design the boom with optimum impedance characteristics in the strip lines, the spacing d between adjacent longitudinal edges of each pair of adjacent strips is precisely established through adjustment of the relative positions of the strips. Through such control of interstrip spacing d, the magnitude of the shunt capacitance between opposed strips is effectively controlled and with it the characteristic impedance of those strip lines. In practice, strips 20-23, inclusive, are simply supported on insulator brackets 25, adjustably if required, so as to provide the proper spaced d between adjacent strips for optimum impedance of both pairs of lines.

The embodiment of the invention shown in FIGS. 1, 2, and 3 with vertically and horizontally polarized sets of dipoles supported and fed by the same boom is exemplary of the simplicity of antenna construction afforded by the practice of this invention. Through provision of removably connectable dipole elements, the antenna may be collapsed for convenience in shipping and quickly reassembled in the field. An eflicient direct electrical coupling as well as a firm mechanical connection between each strip and the associated dipoles are insured because of the flat surfaces of the strips and the advantage of clamping these elements together for the full width of the strip. The oifset positioning of each pair of dipoles relative to the axis of the boom is negligible and has no discernible effect on the antenna performance. The antenna shown in FIGS. 1, 2, and 3 may be operated in either the vertically, horizontally, or circularl polarized modes through appropriate control of switching of the feed lines 30 and 31 in the well-known manner.

The modified form of the invention shown in FIG. 4 is a logarithmically periodic antenna polarized in one plane, the horizontal plane as shown in the drawing. The boom comprises a pair of opposed angle strips 42 and 43 made of conductive material and supported on each other by insulator brackets 44 and 45. A single coaxial line 47 has an outer conductor 47a connected as by brazing to angle strip 43 and the inner conductor 47b is connected at the small end of the boom to the other strip 42 as suggested in broken lines. The antenna elements 14a and 14b are removably connected by clamps 27' and screws 28 to the flat sides of the angle strips as shown and project therefrom in common planes perpendicular to the longitudinal axis of the boom. The spacing d between adjacent edges of the strips is critical as mentioned above and is maintained constant over the length of the antenna for optimum characteristic impedance of the strip transmission line. The angle strips are also dimensionally tapered in a longitudinal direction to provide a tapered boom. The antenna shown in FIG. 4 is electrically the same as the dual linearly polarized antenna of FIG. 3 and has the same advantages of simple construction, ease of connection of the elements, strength and rigidity, and accurate control of electrical parameters of the transmission line for optimum antenna performance.

Modifications and changes to the above-described embodiments of the invention may occur to those skilled in the art without departing from the spirit and principle of the invention. For example, a boom which comprises more than two pairs of strip transmission lines for arrays polarized in more than two planes may readily be constructed. Also, the boom may be constructed in longitudinal sections detachably secured together for further convenience and compactness in storing and shipping the antenna. A dielectric tube with flat sides may be substituted for brackets 25 in order to gain additional strength if desired. The invention is defined in the appended claims.

We claim:

1. An antenna comprising:

an elongated tubular boom having an axis,

said boom comprising at least a pair of longitudinally extending electrically conductive strips defining at least four sides of said boom with adjacent ones of said sides lying in angularly related planes, and means for mechanically coupling and electrically in sulating said strips relative to each other, said strip forming the wall of a unitary tube-like structure.

a plurality of axially spaced antenna elements coupled to said strips, respectively, and

a pair of feed lines electrically connected to the strips,

respectively, whereby electrical energy is transmitted between said antenna elements and the feed lines.

2. The antenna according to claim 1 in which said boom has a rectangular cross-section, each of said strips having a plane surface, at least some of said elements being directly electrically and releasably mechanically connected to the respective strips.

3. The antenna according to claim 2 in which said strips are symmetrically arranged about perpendicular planes parallel to and intersecting along said axis.

4. The antenna according to claim 3 in which each of said strips is a flat plate with a width approximating the transverse dimension of the boom.

5. The antenna according to claim 1 in which at least some of said elements are arranged in axially spaced pairs defining dipoles with the lengths and spacings of axially successive dipoles varying in progressive increments of a predetermined ratio characteristic of a logarithmically periodic array, at least one of said feed lines being electrically connected to the associated strip at one end of the structure, the other feed line being electrically connected to the other strip.

6. An antenna comprising:

an elongated boom having an axis,

said boom comprising at least a pair of longitudinally extending electrically conductive strips and means for mechanically coupling and electrically insulating said strips relative to each other to form a unitary tube-like structure,

a plurality of axially spaced antenna elements coupled to said strips, respectively, and a pair of feed lines electrically connected to the strips, respectively, whereby electrical energy is transmitted between said antenna elements and the feed lines,

at least some of said elements being arranged in axially spaced pairs defining dipoles with the lengths and spacing of axially successive dipoles varying in progressive increments of a predetermined ratio characteristic of a logarithmically periodic structure,

said strips having widths varying uniformly between a minimum and a maximum over the strip length with a constant interstrip spacing.

7. The antenna according to claim '6 in which longitudinally adjacent elements connected to the same strip extend in opposite directions from said strip.

8. The antenna according to claim 6 with a plurality of pairs of said strips and a corresponding number of elements connected to the strips, respectively, at each axial location, and pairs of feed lines connected to said strip pairs, respectively.

9. An antenna comprising:

an elongated boom having an axis,

said boom comprising at least a pair of longitudinally extending electrically conductive strips and means for mechanically coupling and electrically insulating said strips relative to each other to form a unitary tube-like structure,

a plurality of axially spaced antenna elements coupled to said strips, respectively, and

a pair of feed lines electrically connected to the strips,

respectively, whereby electrical energy is transmitted between said antenna elements and the feed lines,

at least some of said elements being arranged in axially spaced pairs defining dipoles with the lengths and spacings of axially successive dipoles varying in progressive increments of a predetermined ratio characteristic of a logarithmically periodic array,

at least one of said feed lines being electrically connected to the associated strip at one end of the structure, the other feed line being electrically connected to the other strip,

said one of said feed lines comprising the inner conductor of a coaxial line coextensive with said tubelike structure, said other feed line being the outer conductor of said coaxial line.

10. The antenna according to claim 9 in which said coaxial line is disposed within said tube-like structure.

11. An antenna comprising:

an elongated tubular boom having an axis and a rectangular cross section,

said boom comprising at least a pair of longitudinally extending electrically conductive strips and means for mechanically coupling and electrically insulating said strips relative to each other to form the wall of a unitary tube-like structure,

each of said strips having two intersecting angularly References Cited related plane surfaces having widths approximating UNITED STATES PATENTS the transverse dimensions of the boom and being symmetrically arranged about'perpendicular planes 2213276 9/1940 9 343*812 2,298,449 11/1942 Balley 343-884 parallel to and mtersectmg along said axis, 5 2 587.146 2/1952 Gross 343 884 a plurahty of axlally spaced antenna elements coupled 3,417,401 12/1968 V61 dhuis 343 822 to said strips, respectively, at least some of said elements being directly electrically ELI LIEBERMAN Primary Examiner and releasably mechanically connected to the respective strips, and 1O a pair of feed lines electrically connected to the strips, respectively, whereby electrical energy is transmitted between said antenna elements and the feed lines.

' U.S. Cl. X.R. 343797, 884 

